Power supply monitoring for an implantable device

ABSTRACT

A method and an apparatus for projecting an end of service (EOS) and/or an elective replacement indication (ERI) of a component in an implantable device and for determining an impedance experienced by a lead associated with the implantable device. An active charge depletion of an implantable device is determined. An inactive charge depletion of the implantable device is determined. A time period until an end of service (EOS) and/or elective replacement indication (ERI) of a power supply associated with the IMD based upon the active charge depletion, the inactive charge depletion, and the initial and final (EOS) battery charges, is determined. Furthermore, to determine the impedance described above, a substantially constant current signal is provided through a first terminal and a second terminal of the lead. A voltage across the first and second terminals is measured. An impedance across the first and second terminals is determined based upon the constant current signal and the measured voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to implantable medical devices, andmore particularly to methods, apparatus, and systems for monitoringpower consumption and impedance characteristics relating to implantablemedical devices.

2. Description of the Related Art

There have been many improvements over the last several decades inmedical treatments for disorders of the nervous system, such as epilepsyand other motor disorders, and abnormal neural discharge disorders. Oneof the more recently available treatments involves the application of anelectrical signal to reduce various symptoms or effects caused by suchneural disorders. For example, electrical signals have been successfullyapplied at strategic locations in the human body to provide variousbenefits, including reducing occurrences of seizures and/or improving orameliorating other conditions. A particular example of such a treatmentregimen involves applying an electrical signal to the vagus nerve of thehuman body to reduce or eliminate epileptic seizures, as described inU.S. Pat. No. 4,702,254 to Dr. Jacob Zabara, which is herebyincorporated by reference in its entirety in this specification.Electrical stimulation of the vagus nerve may be provided by implantingan electrical device underneath the skin of a patient and performing adetection and electrical stimulation process. Alternatively, the systemmay operate without a detection system if the patient has been diagnosedwith epilepsy, and may periodically apply a series of electrical pulsesto the vagus (or other cranial) nerve intermittently throughout the day,or over another predetermined time interval.

Many types of implantable medical devices, such as pacemakers and druginfusion pumps, typically include custom integrated circuits that arecomplex, expensive, and specific to the intended use. These systems alsotypically employ proprietary communications techniques to transferinformation between the implant and an external programmer. The customcircuitry is developed because of the need to keep power consumption ata minimum, to conform to the allowable size for implantable devices, andto support the complexity of the detection and communication techniques,while still supplying the particular intended therapy.

Typically, implantable medical devices (IMDs) involving the delivery ofelectrical pulses to body tissues, such as pacemakers (heart tissue) andvagus nerve stimulators (nerve tissue), comprise a pulse generator forgenerating the electrical pulses and a lead assembly coupled at itsproximal end to the pulse generator terminals and at its distal end toone or more electrodes in contact with the body tissue to be stimulated.One of the key components of such IMDs is the power supply, ordinarily abattery, which may or may not be rechargeable. In many cases surgery isrequired to replace an exhausted battery. To provide adequate warning ofimpending depletion of the battery and subsequent degradation of theoperation of the IMD, various signals may be established and monitored.One such signal is an elective replacement indicator (ERI) that mayindicate that an electrical device component, such as a battery, hasreached a point where replacement or recharging is recommended. Anotherindicator may be an end of service (EOS) signal, which may provide anindication that the operation of the implanted device is at, or near,termination and delivery of the intended therapy can no longer beguaranteed. ERI and EOS are commonly used indicators of the presentstatus of an IMD battery. ERI is intended to be a warning signal of animpending EOS indication, providing sufficient time (e.g., several weeksor months) in typical applications to schedule and perform thereplacement or recharging.

Generally, battery-powered IMDs base the EOS and the ERI signals onbattery voltage and/or battery impedance measurements. One problemassociated with these methodologies is that, for many batterychemistries, these measured battery characteristics do not havemonotonically-changing values with respect to remaining charge. Forexample, lithium/carbon monofluoride (Li/CFx) cells commonly used inneurostimulators and other IMDs have a relatively flat voltage dischargecurve for the majority of their charge life, and present status of thebattery cannot be accurately and unambiguously determined from ameasured battery characteristic.

Another problem associated with this methodology is the variability ofcurrent consumption for a specific device's programmed therapy orcircuitry. This variability, combined with the uncertainty of thebattery's present status prior to ERI or EOS, hinders reliableestimation of the anticipated time until reaching ERI or EOS. Forscheduling purposes, it is desirable to have a constantly available andreliable estimate over all therapeutic parameter ranges and operationsettings of the time until the device will reach EOS, and provide anindication, similar in purpose to ERI, when that time reaches a specificvalue or range.

Impedance measurements are used to assess the integrity of theelectrical leads that deliver the stimulation provided by a pulsegenerator. A change in the impedance across the leads that deliver theelectrical pulses may be indicative either of changes in a patient'sbody or in the electrical leads themselves. For example, damage in thelead, which may be induced by a break in one or more filaments in amultifilament lead wire, or changes in the body tissue where stimulationis delivered, may affect the efficacy of the stimulation therapy.Therefore, it is desirable for changes in the lead impedance, which maybe indicative of various changes or malfunctions, to be accuratelydetected.

For instance, the integrity of the leads that deliver stimulation is ofinterest to insure that the proper therapy dosage is delivered to thepatient. Some IMDs, most notably pacemakers, provide avoltage-controlled output that is delivered to one or more bodylocations (such as the heart). Other IMDs, such as a vagus nervestimulator device developed by Cyberonics, Inc., provide acurrent-controlled output. Generally, however, state-of-the-artmeasurements of lead impedance involve an analysis of the delivery of avoltage signal from a capacitive (C) energy storage component throughthe resistive (R) lead impedance and an examination of the decay of thatsignal based upon a time-constant proportional to the product of theresistance and capacitance (RC). The total equivalent impedance presentat the leads and the known energy source total equivalent capacitancecause a time-constant discharge curve. As the voltage on the capacitanceis discharged through the resistance, the exponential decay of thisvoltage may be monitored to determine the decay time constant RC. Fromthat time constant and an estimate of the known equivalent capacitanceC, the equivalent resistance R presented by the leads may bemathematically estimated. However, this type of measurement may lead toinaccuracies for a number of reasons, including the fact that thedischarging of the voltage signal may be affected by other resistancesand capacitances in the system, the accuracy of the capacitor, the time,voltage, and algorithmic accuracies of the measurement system, and thelike. It would be desirable to have a more efficient and accuratemethod, apparatus, and/or system to measure or assess the impedancepresent at the leads that deliver an electrical stimulation or therapy.

The present invention is directed to overcoming, or at least reducing,the effects of, one or more of the problems set forth above.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a method is provided forprojecting an end of service date and/or elective replacement indicationof a power supply in an implantable medical device, the power supplyhaving an initial electrical charge and a final electrical charge.According to a preferred embodiment, the method comprises determining anactive charge depletion of an IMD, determining an inactive chargedepletion of the implantable device, and determining a time period untilan end of service (EOS) and/or elective replacement indication (ERI) ofa power supply associated with the IMD based upon the active chargedepletion, the inactive charge depletion, and the initial and final(EOS) battery charges.

In another embodiment, a method for projecting an end of service and/oran elective replacement indication of an IMD having a power supply withan initial electrical charge and a final electrical charge comprisesdetermining a current usage rate for at least one future idle period,and determining charge depleted during at least one previous idleperiod. The method also comprises determining a current usage rate forat least one future stimulation period, and determining charge depletedduring at least one previous stimulation period. A total charge depletedby the IMD is determined based upon the charges depleted during the atleast one previous idle period and the at least one previous stimulationperiod, respectively. A total future charge depletion is determinedbased upon the current usage rate during the at least one futurestimulation period and the current usage rate during the at least onefuture idle period. A time period until an end of service (EOS) and/orERI of a power supply (e.g., a battery) of the IMD is determined basedupon the total charge depleted and the total future charge depletion, aswell as the initial and final (EOS) battery charges.

In a further embodiment of the present invention, a method is providedfor projecting an end of service date and/or elective replacementindication of a power supply in an implantable medical device, the powersupply having an initial electrical charge and a final electricalcharge. According to a preferred embodiment, the method comprisesdetermining a charge depletion of an IMD and determining a time perioduntil an end of service (EOS) and/or elective replacement indication(ERI) of a power supply associated with the IMD based upon the chargedepletion and the initial and final (EOS) battery charges.

In a further embodiment of the present invention, a method forprojecting an end of service and and/or elective replacement indicationof an IMD having a power supply with an initial electrical charge and afinal electrical charge comprises determining a previous active depletedcharge of an IMD and determining a future or potential active currentusage rate of the IMD. The method also comprises determining a previousinactive depleted charge of the IMD and determining a future orpotential inactive current usage rate of the IMD. A time period until anEOS and/or ERI of a power supply associated with the implantable deviceis determined based upon the previous active depleted charge, thepotential active current usage rate, the previous inactive depletedcharge, the potential inactive current usage rate, and the initial andfinal (EOS) battery charges.

In another aspect of the present invention, an implantable medicaldevice is provided for projecting an end of service and/or an electivereplacement indication of a power supply in the IMD. The IMD comprises abattery with an initial electrical charge and a final electrical chargeto provide power for at least one operation performed by the implantabledevice. The device further comprises a stimulation unit operativelycoupled to the battery, the stimulation unit providing a stimulationsignal to at least one body location. The stimulation unit preferablycomprises an electrical pulse generator, but may alternatively comprisea drug pump, a magnetic field generator, a mechanical vibrator element,or other device for stimulating body tissue. The IMD also preferablycomprises a controller operatively coupled to the stimulation unit andthe battery. The controller is adapted to determine an active currentusage rate and an inactive current usage rate of the IMD, as well as anactive electrical charge depleted by the battery during stimulation andan inactive electrical charge depleted by the battery during inactiveperiods in which no electrical stimulation is provided to the patient.The controller is further adapted to determine a time period until anend of service of a power supply associated with the IMD based upon theactive current usage rate and the inactive current usage rates, theactive and inactive electrical charges depleted, and the initial andfinal electrical charges of the battery.

In still another aspect, the present invention comprises an IMD forprojecting an EOS and/or an ERI of a battery. The IMD comprises abattery with an initial and a final (EOS) electrical charge, astimulation unit providing an electrical stimulation signal, and acontroller. The controller is adapted to determine first and secondactive current usage rates for current usage in a first stimulationtherapy and a second stimulation therapy, respectively. The controlleris also adapted to determine first and second inactive (i.e.,non-stimulating) current usage rates in a first inactive mode and asecond inactive mode, respectively. In addition, the controller isadapted to determine an active electrical charge depleted by the batteryduring stimulation and an inactive electrical charge depleted duringinactive periods. The controller also determines a time period until anEOS and/or an ERI of the battery, based upon the first and second activecurrent usage rates, the first and second inactive current usage rates,the active and inactive electrical charges depleted, and the initial andfinal electrical battery charges.

In another aspect of the present invention, a system is provided forprojecting an EOS and/or an ERI of a power supply of an IMD. The systemcomprises an external device (i.e., a device outside the body of thepatient) for performing remote communications with the IMD, and the IMDis also capable of communicating with the external device as well asdelivering a stimulation signal to the patient. The IMD comprises abattery to provide power for delivering the stimulation signal, acommunications unit to provide communications between the externaldevice and the IMD, and a stimulation unit operatively coupled to thebattery for providing a stimulation signal. The system also comprises acontroller operatively coupled to the stimulation unit and to thebattery. The controller comprises a charge depletion circuit fordetermining both an active charge depletion and an inactive chargedepletion of the IMD. The controller further comprises an EOS/ERIcircuit for determining a time period until an end of service and/or anelective replacement indication of a power supply associated with theimplantable device, based upon the active charge depletion, the inactivecharge depletion, and the original and EOS battery charges.

In yet another aspect of the present invention, a computer readableprogram storage device encoded with instructions is provided forprojecting an end of service and/or an elective replacement indicationof a power supply in an IMD. The computer readable program storagedevice is encoded with instructions that, when executed by a computer,determine an active charge depletion and an inactive charge depletion ofthe IMD, and also determines a time period until an end of serviceand/or an elective replacement indication of a power supply associatedwith the IMD based upon the determined active charge depletion, thedetermined inactive charge depletion, and the initial and final batterycharges.

In another aspect of the present invention, a method is provided fordetermining an impedance presented by a lead associated with an IMD. Inthe method, a substantially constant current signal is provided througha first terminal and a second terminal of the lead. A voltage across thefirst and second terminals is measured, and an impedance across thefirst and second terminals is determined based upon the constant currentsignal provided and the measured voltage.

In another aspect of the present invention, an IMD is provided thatcomprises circuitry for determining an impedance presented by a leadassociated with the IMD. The IMD comprises an amplifier circuit forproviding a substantially constant current signal through a firstterminal and a second terminal of a lead. The IMD further comprises avoltage measurement unit to measure a voltage across the first andsecond terminals. The implantable device additionally comprises animpedance determination unit to determine an impedance between the firstand second terminals based upon the constant current signal and thevoltage.

In another aspect of the present invention, a system is provided fordetermining an impedance experienced by a lead associated with an IMD.The system comprises an external device communicating with the IMD, andthe IMD is in turn adapted to communicate with the external device andto deliver a stimulation signal to a lead coupled to the IMD. The IMDcomprises an amplifier circuit for providing a substantially constantcurrent signal through a first terminal and a second terminal of thelead. The IMD also includes a voltage measurement unit to measure avoltage across the first and second terminals, and an impedancedetermination unit to determine an impedance between the first andsecond terminals based upon the constant current signal and the measuredvoltage. The IMD may also include a communications unit forcommunicating data relating to the impedance determination to theexternal device.

In yet another aspect of the present invention, a computer readableprogram storage device encoded with instructions is provided fordetermining an impedance experienced by a lead coupled to an IMD. Thecomputer readable program storage device is encoded with instructionsthat when executed by a computer, preferably within the IMD, provides asubstantially constant current signal through a first terminal and asecond terminal of the lead, measures a voltage across the first andsecond terminals, and determines an impedance across first and secondterminals based upon the constant current signal and the voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich like reference numerals identify like elements, and in which:

FIG. 1A is a stylized diagram of an implantable medical device suitablefor use in the present invention implanted into a patient's body;

FIG. 1B is a stylized diagram of another embodiment of an implantablemedical device suitable for use in the present invention implanted intoa patient's body;

FIG. 1C illustrates an implantable medical device suitable for use inthe present invention, showing the header and electrical connectors forcoupling the device to a lead/electrode assembly;

FIG. 1D shows a lead and electrodes suitable for use in the presentinvention attached to a vagus nerve of a patient;

FIG. 2 is a block diagram of an implantable medical device and anexternal unit that communicates with the implantable medical device, inaccordance with one illustrative embodiment of the present invention;

FIG. 3 is a stylized diagram of an output current signal provided by theimplantable medical device of FIGS. 1 and 2, provided to illustratecertain stimulation parameters in accordance with one illustrativeembodiment of the present invention;

FIG. 4 is a flowchart representation of a method of providing a warningsignal relating to a power supply of the implantable medical device, inaccordance with one illustrative embodiment of the present invention;

FIG. 5 is a flowchart representation of a method of performing acalibration of a charge depletion tabulation, in accordance with oneillustrative embodiment of the present invention;

FIG. 6 is a more detailed flowchart illustrating a method of performingthe charge depletion calculation indicated in FIG. 4, in accordance withone illustrative embodiment of the present invention;

FIG. 7 is a more detailed flowchart illustrating a method of performingan end-of-service (EOS) and/or an elective replacement indication (ERI)determination, as indicated in FIG. 4, in accordance with oneillustrative embodiment of the present invention;

FIG. 8, is a block diagram of the stimulation unit shown in FIG. 2, inaccordance with one illustrative embodiment of the present invention;

FIG. 9 is a block diagram of the impedance measurement unit shown inFIG. 2, in accordance with one illustrative embodiment of the presentinvention;

FIG. 10 is a flowchart of a method of performing an impedancemeasurement, in accordance with one illustrative embodiment of thepresent invention; and

FIG. 11 is a flowchart of a method of performing a calibration of an A/Dconverter used for impedance measurement, in accordance with oneillustrative embodiment of the present invention.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Illustrative embodiments of the invention are described herein. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. In the development of any such actualembodiment, numerous implementation-specific decisions must be made toachieve the design-specific goals, which will vary from oneimplementation to another. It will be appreciated that such adevelopment effort, while possibly complex and time-consuming, wouldnevertheless be a routine undertaking for persons of ordinary skill inthe art having the benefit of this disclosure.

Embodiments of the present invention provide methods and apparatus formonitoring and/or estimating the electrical charge depletion of animplantable medical device (IMD). Estimating battery life may be basedupon estimated future charge depletion and actual past charge depletion.Embodiments of the present invention provide for an elective replacementindicator (ERI) signal to provide a warning for performing an electricaldiagnostic operation upon the IMD. This electrical diagnostic operationmay include replacing an electrical component in the IMD, performingadditional evaluation(s) of the operation of the IMD, replacing orrecharging a power source of the IMD, and the like. A more detaileddescription of an IMD suitable for use in the present invention isprovided in various figures and the accompanying description below.

Generally, IMDs contain power storage devices or battery units toprovide power for the operations of the IMD. Embodiments of the presentinvention determine an estimated usable life remaining in the batteryunit based upon determining initial and final battery charges, chargedepleted by operations of the IMD, and a future depletion rate.Embodiments of the present invention may be performed in a standalonemanner within the IMD itself, or in conjunction with an external devicein communication with the IMD. Utilizing embodiments of the presentinvention, an end of service (EOS) signal or an ERI signal may beprovided, indicating that the IMD is at or near termination ofoperations and/or the battery power has reached a level at whichreplacement should be considered to avoid interruption or loss oftherapy to the patient.

Other embodiments of the present invention provide for determining thelead impedance. This process involves determining the voltage across alead associated with the IMD, based upon the delivery of a constantcurrent signal. The impedance may be measured on demand or atpredetermined periodic intervals to detect significant changes inimpedance across the leads of the IMD. Changes in the impedance may belogged and time-stamped, and saved in a memory in the IMD for diagnosticconsiderations. Voltage and current measurements associated with the IMDmay be calibrated using various impedance measurements in order toenhance the accuracy of lead impedance measurements.

FIGS. 1A-1D illustrate a generator 110 having main body 112 comprising acase or shell 121 (FIG. 1A) with a connector 116 (FIG. 1C) forconnecting to leads 122. The generator 110 is implanted in the patient'schest in a pocket or cavity formed by the implanting surgeon just belowthe skin (indicated by a dotted line 145), similar to the implantationprocedure for a pacemaker pulse generator. A stimulating nerve electrodeassembly 125, preferably comprising an electrode pair, is conductivelyconnected to the distal end of an insulated electrically conductive leadassembly 122, which preferably comprises a pair of lead wires (one wirefor each electrode of an electrode pair). Lead assembly 122 is attachedat its proximal end to the connector 116 on case 121. The electrodeassembly is surgically coupled to a vagus nerve 127 in the patient'sneck. The electrode assembly 125 preferably comprises a bipolarstimulating electrode pair (FIG. 1D), such as the electrode pairdescribed in U.S. Pat. No. 4,573,481 issued Mar. 4, 1986 to Bullara.Persons of skill in the art will appreciate that many electrode designscould be used in the present invention. The two electrodes arepreferably wrapped about the vagus nerve, and the electrode assembly 125is preferably secured to the nerve 127 by a spiral anchoring tether 128(FIG. 1D) such as that disclosed in U.S. Pat. No. 4,979,511 issued Dec.25, 1990 to Reese S. Terry, Jr. and assigned to the same assignee as theinstant application. Lead assembly 122 is secured, while retaining theability to flex with movement of the chest and neck, by a sutureconnection 130 to nearby tissue.

In one embodiment, the open helical design of the electrode assembly 125(described in detail in the above-cited Bullara patent), which isself-sizing and flexible, minimizes mechanical trauma to the nerve andallows body fluid interchange with the nerve. The electrode assembly 125preferably conforms to the shape of the nerve, providing a lowstimulation threshold by allowing a large stimulation contact area withthe nerve. Structurally, the electrode assembly 125 comprises twoelectrode ribbons (not shown), of a conductive material such asplatinum, iridium, platinum-iridium alloys, and/or oxides of theforegoing. The electrode ribbons are individually bonded to an insidesurface of an elastomeric body portion of the two spiral electrodes125-1 and 125-2 (FIG. 1D), which may comprise two spiral loops of athree-loop helical assembly. The lead assembly 122 may comprise twodistinct lead wires or a coaxial cable whose two conductive elements arerespectively coupled to one of the conductive electrode ribbons 125-1and 125-2. One suitable method of coupling the lead wires or cable tothe electrodes comprises a spacer assembly such as that disclosed inU.S. Pat. No. 5,531,778, although other known coupling techniques may beused. The elastomeric body portion of each loop is preferably composedof silicone rubber, and the third loop 128 (which typically has noelectrode) acts as the anchoring tether 128 for the electrode assembly125.

In certain embodiments of the invention, eye movement sensing electrodes133 (FIG. 1B) may be implanted at or near an outer periphery of each eyesocket in a suitable location to sense muscle movement or actual eyemovement. The electrodes 133 may be electrically connected to leads 134implanted via a catheter or other suitable means (not shown) andextending along the jawline through the neck and chest tissue to thestimulus generator 110. When included in systems of the presentinvention, the sensing electrodes 133 may be utilized for detectingrapid eye movement (REM) in a pattern indicative of a disorder to betreated, as described in greater detail below.

Alternatively or additionally, EEG sensing electrodes 136 may optionallybe implanted in spaced apart relation through the skull, and connectedto leads 137 implanted and extending along the scalp and temple and thenalong the same path and in the same manner as described above for theeye movement electrode leads. Electrodes 133 and 137, or other types ofsensors, may be used in some embodiments of the invention to triggeradministration of the electrical stimulation therapy to the vagus nerve127 via electrode assembly 125. Use of such sensed body signals totrigger or initiate stimulation therapy is hereinafter referred to as afeedback loop mode of administration. Other embodiments of the presentinvention utilize a continuous, periodic or intermittent stimulus signalapplied to the vagus nerve (each of which constitutes a form ofcontinual application of the signal) according to a programmed on/offduty cycle without the use of sensors to trigger therapy delivery. Thistype of delivery may be referred to as a prophylactic therapy mode. Bothprophylactic and feedback loop administration may be combined ordelivered by a single IMD according to the present invention. Either orboth modes may be appropriate to treat the particular disorder diagnosedin the case of a specific patient under observation.

The pulse generator 110 may be programmed with an external computer 150using programming software of the type copyrighted by the assignee ofthe instant application with the Register of Copyrights, Library ofCongress, or other suitable software based on the description herein,and a programming wand 155 to facilitate radio frequency (RF)communication between the computer 150 (FIG. 1A) and the pulse generator110. The wand 155 and software permit noninvasive communication with thegenerator 110 after the latter is implanted. The wand 155 is preferablypowered by internal batteries, and provided with a “power on” light toindicate sufficient power for communication. Another indicator light maybe provided to show that data transmission is occurring between the wandand the generator.

FIG. 2 illustrates one embodiment of an IMD 200 (which may comprisepulse generator 110) for performing neurostimulation in accordance withembodiments of the present invention. In one embodiment, the implantablemedical device 200 comprises a battery unit 210, a power-sourcecontroller 220, a stimulation controller 230, a power regulation unit240, a stimulation unit 250, an impedance measurement unit 265, a memoryunit 280 and a communication unit 260. It will be recognized that one ormore of the blocks 210-280 (which may also be referred to as modules)may comprise hardware, firmware, software, or any combination of thethree. The memory unit 280 may be used for storing various programcodes, starting data, and the like. The battery unit 210 comprises apower-source battery that may be rechargeable or non-rechargeable. Thebattery unit 210 provides power for the operation of the IMD 200,including electronic operations and the stimulation function. Thebattery unit 210, in one embodiment, may be a lithium/thionyl chloridecell or, more preferably, a lithium/carbon monofluoride (Li/CFx) cell.The terminals of the battery unit 210 are preferably electricallyconnected to an input side of the power-source controller 220 and thepower regulation unit 240.

The power-source controller 220 preferably comprises circuitry forcontrolling and monitoring the flow of electrical power to variouselectronic and stimulation-delivery portions of the IMD 200 (such as themodules 230-265 and 280 illustrated in FIG. 2). More particularly, thepower-source controller 220 is capable of monitoring the powerconsumption or charge depletion of the implantable medical device 200and is capable of generating the ERI and the EOS signals. Thepower-source controller 220 comprises an active charge-depletion unit222, an inactive charge-depletion unit 224, and an ERI/EOS calculationunit 226. The active charge-depletion unit 222 is capable of calculatingthe charge depletion rate of the implantable medical device 200 duringactive states, and may comprise sub-units to calculate the chargedepletion rates of a plurality of active states comprising differentcharge depletion rates. The active state of the implantable medicaldevice 200 may refer to a period of time during which a stimulation isdelivered by the implantable medical device 200 to body tissue of thepatient according to a first set of stimulation parameters. Other activestates may include states in which other activities are occurring, suchas status checks and/or updates, or stimulation periods according to asecond set of stimulation parameters different from the first set ofstimulation parameters. The inactive charge-depletion unit 224 iscapable of calculating the charge depletion rate of the implantablemedical device 200 during inactive states. Inactive states may alsocomprises various states of inactivity, such as sleep mode, wait modes,and the like. The ERI/EOS calculation unit 226 is capable of performingcalculations to generate an ERI signal and/or an EOS signal. One or moreof the active charge-depletion unit 222, the inactive charge-depletionunit 224, and/or the ERI/EOS calculation unit 226 may be hardware,software, firmware, and/or any combination thereof.

The power regulation unit 240 is capable of regulating the powerdelivered by the battery unit 210 to particular modules of the IMD 200according to their needs and functions. The power regulation unit 240may perform a voltage conversion to provide appropriate voltages and/orcurrents for the operation of the modules. The power regulation unit 240may comprise hardware, software, firmware, and/or any combinationthereof.

The communication unit 260 is capable of providing transmission andreception of electronic signals to and from an external unit 270. Theexternal unit 270 may be a device that is capable of programming variousmodules and stimulation parameters of the IMD 200. In one embodiment,the external unit 270 is a computer system that is capable of executinga data-acquisition program. The external unit 270 is preferablycontrolled by a healthcare provider such as a physician, at a basestation in, for example, a doctor's office. The external unit 270 may bea computer, preferably a handheld computer or PDA, but may alternativelycomprise any other device that is capable of electronic communicationsand programming. The external unit 270 may be used to download variousparameters and program software into the IMD 200 for programming theoperation of the implantable device. The external unit 270 may alsoreceive and upload various status conditions and other data from the IMD200. The communication unit 260 may comprise hardware, software,firmware, and/or any combination thereof. Communications between theexternal unit 270 and the communication unit 260 may occur via awireless or other type of communication, illustrated generally by line275 in FIG. 2.

Stimulation controller 230 defines the stimulation pulses to bedelivered to the nerve tissue according to parameters and waveforms thatmay be programmed into the IMD 200 using the external unit 270. Thestimulation controller 230 controls the operation of the stimulationunit 250, which generates the stimulation pulses according to theparameters defined by the controller 230 and in one embodiment providesthese pulses to the connector 116 for delivery to the patient via leadassembly 122 and electrode assembly 125 (see FIG. 1A). Variousstimulation signals provided by the implantable medical device 200 mayvary widely across a range of parameters. The Stimulation controller 230may be hardware, software, firmware, and/or any combination thereof.

FIG. 3 illustrates the general nature, in idealized representation, ofan output signal waveform delivered by the output section of a pulsegenerator 110 (such as stimulation unit 250 shown in FIG. 2) to leadassembly 122 and electrode assembly 125 in an embodiment of the presentinvention. This illustration is presented principally for the sake ofclarifying terminology, including the parameters of signal on-time,off-time, frequency, pulse width, and current. In the treatment of aneuropsychiatric disorder in an exemplary implementation, thestimulation unit 250 of the IMD 200 delivers pulses having a desiredoutput signal current and frequency, with each pulse having a desiredoutput signal pulse width. The pulses are delivered for the duration ofthe output signal on-time (stimulation period), and are followed by theoutput signal off-time during which no output signal is delivered (idleperiod). This periodic stimulation reduces the symptoms of theneuropsychiatric disorder. Stimulation parameters suitable for treatmentof a variety of medical conditions can be found in the followingpatents: U.S. Pat. No. 4,702,254, U.S. Pat. No. 5,025,807, U.S. Pat. No.4,867,164, and U.S. Pat. No. 6,622,088 (epilepsy); U.S. Pat. No.5,188,104 and U.S. Pat. No. 5,263,480 (eating disorders); U.S. Pat. No.5,215,086 (migraine headaches); U.S. Pat. No. 5,231,988 (endocrinedisorders); U.S. Pat. No. 5,269,303 (dementia); U.S. Pat. No. 5,299,569and U.S. Pat. No. 6,622,047 (neuropsychiatric disorders); U.S. Pat. No.5,330,513 and U.S. Pat. No. 6,721,603 (pain); U.S. Pat. No. 5,335,657(sleep disorders); U.S. Pat. No. 5,540,730 (motility disorders); U.S.Pat. No. 5,571,150 (coma); U.S. Pat. No. 5,707,400 (refractoryhypertension); U.S. Pat. No. 6,587,719 and U.S. Pat. No. 6,609,025(obesity); U.S. Pat. No. 6,622,041 (congestive heart failure). Each ofthe foregoing patents is hereby incorporated by reference herein in itsentirety.

In one embodiment of the invention, the IMD 200 determines EOS and ERIvalues by using a known initial battery charge (Q₀) and a predeterminedEOS battery charge (Q_(EOS)) indicative of the end of useful batteryservice, together with the charge actually depleted (Q_(d)) by the IMD(calculated from the current usage rates for idle and stimulationperiods (r_(i) and r_(s)), and the length of the respective idle andstimulation periods), to calculate for a desired time point how muchuseful charge remains on the battery (Q_(r)) until the EOS charge isreached, and how long at projected current usage rates the device canoperate until EOS or ERI. Once the charge actually depleted by operationof the device (Q_(d)) is known, the current usage rates are then appliedto the remaining useful charge Q_(r) to determine the time remaininguntil EOS and/or ERI.

The present invention allows EOS and ERI determinations to be madewithout measurements or calculations of internal battery impedance orother battery parameters. Instead, the device maintains a precise recordof the current used during idle and stimulation periods, and subtractsthe charge represented by the current used from the total availablebattery charge to determine the charge remaining on the battery. Becausethe relative duration of stimulation and idle periods are determined bythe stimulation programming parameters of the IMD, a determination ofEOS and ERI can be calculated in a straightforward manner based upon thecurrent usage rates associated with the programming parameters.

Consistent with the foregoing, FIG. 4 provides a flowchart depiction ofa method for determining the remaining time to EOS and/or ERI based onknown or determined IMD characteristics such as battery charge andcurrent usage rates. In one embodiment, the current usage of the IMD 200is calibrated during manufacture (step 410). Current drawn by the IMDfrom the battery is defined as electrical charge per unit time. Thetotal charge depleted from the battery as a result of the operations ofthe IMD may be determined by multiplying each distinct current rate usedby the IMD by its respective time used. In one embodiment, as part ofthe calibration, during manufacturing, a power supply capable ofgenerating known currents and voltages may be used to characterize thepower consumption or current depletion of the implantable medical device200 during its stimulation and idle modes The power consumption datathus obtained is preferably stored in a memory of the IMD.

Once the charge usage characteristics of the IMD are known, the batterymay be subsequently installed into the implantable medical device 200for operation and therafter a record of power consumed by theimplantable medical device 200 is maintained. In a particularembodiment, the calibration step 410 involves calibration of currentusage for idle periods (r_(i)) and stimulation periods (r_(s)) of thedevice. Current may thus be used as a proxy value for electrical chargedepletion, and the calibration step allows a precise determination ofthe amount of electrical charge used by the device after implantation.As used herein, the terms “depletion rate,” “consumption rate,” and“usage rate” may be used interchangeably and refer to the rate at whichelectrical charge is depleted from the battery. However, as noted above,current may be used as a proxy for electrical charge, and where this isthe case, current rates r_(i) and r_(s) may also be referred to as“current usage,” “current rate,” “current consumption,” “chargedepletion,” “depletion rate” or similar terms.

As previously noted, the IMD 200 has a number of settings and parameters(e.g., current, pulse width, frequency, and on-time/off-time) that canbe changed to alter the stimulation delivered to the patient. Thesechanges result in different current usage rates by the IMD 200. Inaddition, circuit variations from device to device may also result indifferent current usage rates for the same operation. Calculations andestimations are preferably performed during the manufacturing process inorder to calibrate accurately and precisely the current usage rates ofthe IMD 200 under a variety of stimulation parameters and operatingconditions. A calibration of the current usage rates and a determinationof the charge present on the battery at the time of implant allow a moreaccurate assessment of actual and predicted charge depletion after theIMD 200 is implanted. The initial charge on the battery may include asafety factor, i.e., the charge may be a “minimum charge” that allbatteries are certain to possess, even though many individual batteriesmay have a significantly greater charge. Nothing herein precludes adetermination of a unique initial charge for each individual battery.However, it will be recognized that such individual determinations maynot be economically feasible. A more detailed illustration anddescription of the step (410) of calibrating current usage andinitializing the battery charge for the implantable medical device 200is provided in FIG. 5 and the accompanying description below.

After calibrating the current usage characteristics of the IMD 200, theIMD may be implanted and subsequently a charge depletion calculation isperformed (step 420). This calculation may be performed by the IMDitself, the external unit 270, or by both, and includes determining theactual electrical charge depleted from the battery 210 and estimatingfuture current usage (i.e., depletion rates), which are then used tocalculate an elective replacement indication (ERI) and/or an end ofservice (EOS) signal (step 430). A more detailed illustration anddescription of the step 420 of calculating the electrical chargedepleted is provided in FIG. 6 and the accompanying description below.In step 430 an estimated time until an elective replacement indicationwill be generated and/or the estimated time until the end of service arecalculated utilizing the initial battery charge, the actual chargeconsumed and the estimated future charge depletion calculated in lightof the calibration performed during manufacture. A more detaileddescription and illustration of the step 430 of calculating the time toERI and/or EOS is provided in FIG. 7 and the accompanying descriptionbelow.

Referring now to FIG. 5, a flowchart diagram is provided depicting ingreater detail the step 410 (FIG. 4) of calibrating and initializing theIMD 200 during manufacturing. In one embodiment, the current rates forthe IMD 200 during stimulation are calibrated (block 510). Duringmanufacturing, several different combinations of measurements may becalibrated. More specifically, measurements of charge depletion relatingto different types of pulses (i.e., pulses having different stimulationparameters) are calibrated to ensure that current usage measurements forthe IMD are accurate over a wide range of stimulation parameters. Inother words, various pulses having a range of current amplitudes, pulsewidths, frequencies, duty cycles and/or lead impedances into which thepulses are delivered are used to calibrate the measurement of currentusage during stimulation to establish a baseline of the measurement ofcharge depletion for various types of pulses. All operational variablesrelating to or affecting the current usage rates of the IMD may beconsidered.

More particularly, during manufacture of the IMD 200, severalcombinations of data points relating to various current rates resultingfrom various combinations of pulse parameters are used in one embodimentto generate a linear equation that relates various pulse parameters tocurrent rate, which may then be used to determine charge depletion. Forexample, for a first stimulation, pulses of a certain frequency areprovided and for a second stimulation, the frequency of the pulses usedmay be doubled. Therefore, the estimated current usage rate for thesecond stimulation may be estimated to be approximately double that ofthe power consumption or charge depleted due to the first stimulation.As another example, a first stimulation may be of a first pulse widthand a second stimulation may be of a pulse width that is double that ofthe width of the first pulse. Therefore, a relationship between thepulse width to the current consumption of the second pulse may beestimated to be approximately double that of the current usage rate ofthe first pulse. In one embodiment, a graph may be generated using thevarious types of stimulation versus the current consumption associatedwith that stimulation.

As yet another example, a first stimulation pulse may have a firstcurrent amplitude and a second stimulation may have a current amplitudethat is double that of the first stimulation pulse. Therefore, thecurrent consumption of the second stimulation pulse may be estimated tobe approximately double that of the current consumption of the firststimulation pulse. The power consumption is directly proportional to thecurrent consumption. Therefore, a relationship of a pulse parameter tocurrent usage rate may be estimated or measured such that aninterpolation may be performed at a later time based upon the linearrelationship developed during the calibration of the power consumptionduring stimulation. It may be appreciated that the relationships of somepulse parameters to current usage rate may not be a simple linearrelationship, depending upon such pulse characteristics as the type ofpulse decay (i.e., square wave, exponential decay), for example.Nevertheless, calibration of current usage rate for various pulseparameters may be performed by routine calculation or experiment forpersons of skill in the art having the benefit of the presentdisclosure.

Referring again to FIG. 5, current usage during an idle (i.e.,non-stimulating) period is calibrated in step 520. From the idle currentconsumption and the stimulation current consumption calibration, theoverall current consumption may be modeled based upon programmedsettings. It should be noted that while the invention as shown in thedrawings describes a device having two current usage patterns associatedwith an idle period and a stimulation period, such a two-stateembodiment is described solely for clarity, and more complex embodimentsare possible involving a third state such as, by way of nonlimitingexample, a current usage rate associated with electrical sensing of thelead electrodes, which may be defined by a third current rate r₃.Four-state or even higher state embodiments are possible, although wherethe differences in current usage rates are small, or where a particularcurrent usage rate comprises only a tiny fraction of the overall time ofthe device, the complexity required to implement and monitor the timesuch current rates are actually used by the device may render the deviceimpractical. These multi-state embodiments may be implemented ifdesired, however, and remain within the scope and spirit of the presentinvention.

Using the calibration of current usage during stimulation periods (step510) and idle periods (step 520), a calculation may optionally be madeto initialize the charge depleted, if any, during manufacturingoperations, such as the charge depleted during testing of the deviceafter assembly (block 530). In a preferred embodiment, all of thecalibrations are performed with a calibrated current source device, andnot a battery, and in this case there is no charge depletion duringmanufacturing operations. In another embodiment, the amount of chargedepleted during manufacturing may small, in which case theinitialization procedure may also be omitted. The calibration and/orinitialization steps of FIG. 5 allow the IMD 200, via power-sourcecontroller 220, to maintain a running tally of how much charge has beendepleted from the device. When the battery unit 210 is first insertedinto the implantable medical device 200, the charge depleted isgenerally initialized to zero so that a running tabulation may beginfrom zero for maintaining a running tally of the charge depleted fromthe battery over the life of the implantable medical device 200. In oneembodiment, the charge depleted tally is incremented throughout theoperating life of the device and at any point the running tally may besubtracted from the known initial charge of the battery to determine theremaining charge. In an alternative embodiment, the charge depletedtally could be initialized to the value of the battery initial chargeand the tally decremented throughout the device operation and directlyused as the remaining charge. In either implementation, informationrelating to the baseline charge remaining on the battery at the end ofmanufacturing may be retained to calculate the estimated time to EOS orERI.

Turning now to FIG. 6, a flowchart depiction of the step 420 ofcalculating charge depleted by the device is provided in greater detail.For simplicity, only the two-current state of a single idle period and asingle stimulation period is shown. Embodiments having additionalcurrent usage rates are included in the present invention. The IMD 200may determine a current depletion rate r_(i) for idle periods (block610). The rate is preferably stored in memory. In one embodiment, thedetermination is made by the IMD 200 after implantation. In a preferredembodiment, the idle current depletion rate may be a rate determinedduring manufacturing (i.e., a rate calibrated in step 520) and stored inthe memory 280. An idle period is defined as a time period when theimplantable medical device 200 is not performing active stimulation,i.e., is not delivering a stimulation pulse to the electrodes. Variouselectronic functions, such as tabulation and calculation of numbers orexecution of various software algorithms within the IMD 200 may takeplace during the idle period.

As noted, the current rate r_(i) during idle periods 610 may bepredetermined during the manufacturing process (step 520) and mayinclude various considerations, such as the power consumption of theoperation of various electronics in the implantable medical device 200,even though no active stimulation may be taking place during that timeperiod. However, since the implantable medical device 200 may beoccasionally reprogrammed while still implanted inside a patient's body,the number and duration of idle periods may vary according to the dutycycle and frequency of the stimulation pulses. Therefore, the IMD 200(e.g., via the power source controller 220 in the device) may maintain arunning tabulation of the idle periods, and for each idle period acertain amount of charge depleted during the idle period (i.e., offtime) is tabulated and stored in memory 280 (step 620).

It will be appreciated that the depleted charge may be obtained in anumber of different ways, each within the scope of the presentinvention. Specifically, the total time of all idle periods sinceimplantation, initialization, or since a previous idle power depletioncalculation, may be maintained as a running total idle time in memory,or alternatively a running tally of charge depleted during idle periodsmay be maintained. While these values are different numerically, theyare directly related by simple equations as discussed more fullyhereinafter. At an update time, the total idle time may be periodicallyaccessed and multiplied by the idle period current usage rate todetermine the total power depleted during idle periods sinceimplantation, initialization, or the previous calculation.

The IMD 200 may also maintain in memory 280 a tabulation of currentusage rates (i.e., charge depletion) for a wide range of stimulationsettings (step 630). In another embodiment, theoretical charge depletioncalculations relating to particular types of stimulation may be providedto the IMD 200. The stimulation parameter settings may then be used bythe device to maintain a running tabulation of the charge depletedduring stimulation periods using a current usage rate r_(s) calculatedfrom the pulse width, pulse amplitude, pulse frequency, and otherparameters which may impact the current usage rate. This method providesspecific current usage rates for a variety of stimulation parametersettings and lead impedances without requiring the storage of currentusage rates for all possible stimulation parameter settings and leadimpedances.

In one embodiment, the charge depleted may be stored in micro-ampseconds; however, various other measurement units may be utilized. Inone embodiment, the IMD 200 itself may be capable of calculating thecurrent usage rate for a particular combination of programmed outputsettings based upon a known relationship between current usage rates anddifferent combinations of programmed settings. The relationship may thenbe used to interpolate a particular current usage rate for a particularcombination of programmed output settings. However, in order to reducethe computation load on the device, some or all of these calculations,including the interpolation, are preferably performed by an externalprogrammer 270. Therefore, upon programming or performing routinemaintenance of the implantable medical device 200, the external unit 270may perform the calculations to determine the current usage rate duringfuture stimulation cycles based upon the settings implemented during theprogramming or maintenance operation.

For example, if the stimulation for a particular patient is set to aparticular pulse width, the external device 270 may factor in thecalibration data and determine a current usage rate for a particular setof stimulation settings. Therefore, for each stimulation period, thecharge that is depleted is tabulated for the stimulation period(“on-time”) by multiplying the stimulation time by the current usagerate and a running tabulation is maintained (block 640). For example, ifthe predetermined current usage rate for each second of stimulation at aparticular combination of parameter settings is 100 microamps, and thestimulation is 30 seconds long, a calculation is made by multiplying the30 second time period for the stimulation, by the 100 microamps toarrive at 3000 micro amp seconds of charge consumed, which is then addedto a running charge consumption tally.

As illustrated in FIG. 6, the sum of the tabulations of the chargedepleted for the idle period (off-time or inactive period; step 620) andthe charge depleted for the stimulation period (on-time or activeperiod; step 640) are added to arrive at a total charge depleted by theIMD 200 (block 650). It will be appreciated that the sum of idle periodand stimulation charge depletion may occur at the conclusion of one ormore cycles of idle period and stimulation period, or continuouslythroughout idle periods and stimulation periods. Occasionally during theoperational life of the IMD 200, various stimulation parameters may bechanged to provide different types of stimulation. However, utilizingthe steps described herein, a running tally (or a periodically updatedtally) of the charge depletion is maintained, such that even when thestimulation settings change, the device maintains a substantiallyaccurate reflection of the actual charge that has been depleted by theIMD 200, and future depletion calculations are based on the depletionrate for the newly programmed settings.

The memory 280 may store the results of the charge calculations (step660). The data stored may include both the current usage rates for idleand stimulation periods of the IMD 200, as well as the total chargedepleted. This data may be utilized by the IMD 200 and/or external unit270 to determine various aspects of the device, including the amount ofremaining battery life.

The calculations associated with steps 620, 640 and 650 may be expressedmathematically. In particular, the total charge available from thebattery Q_(tot) after it is placed in the IMD 200 may be represented asthe difference between an initial battery charge Q₀ and the EOS batterycharge Q_(EOS), as expressed in Equation 1.Q _(tot) =Q ₀ −Q _(EOS)  Equation 1The charge depleted by the IMD 200 during idle periods Q_(i) (step 620)may be expressed as the idle period current usage rate r_(i) multipliedby the total duration of all idle periods Δt_(i) according to equation2.Q ₁ =r _(i) ×ΣΔt _(i)  Equation 2Where multiple idle rates are present, the above equation will be solvedfor each idle current usage rate and the results summed to obtain Q_(i).Similarly, the charge depleted during stimulation periods Q_(s) (step640) may be expressed as the stimulation period current usage rate r_(s)multiplied by the total duration of all stimulation periods Δt_(s)according to equation 3.Q _(s) =r _(s) ×ΣΔt _(s)  Equation 3Again, where multiple stimulation rates are used the equation will besolved for each stimulation rate and the results summed. The totalcharge depleted Q_(d) is the sum of Q_(i) and Q_(s), as shown inequation 4.Q _(d) =Q _(i) +Q _(s)  Equation 4.Finally, the charge remaining until EOS (Q_(r)) at any arbitrary pointin time is the difference between the total energy or charge availableQ_(tot) and the charge actually depleted from the battery Q_(d) at thatsame timepoint, as expressed in equation 5 (step 650).Q _(r) =Q _(tot) −Q _(d)  Equation 5This may be accomplished by counters that record the amount of time thedevice uses the idle current usage rate(s) and the stimulation currentusage rate(s), respectively, which are then multiplied by the applicablecurrent usage rate to obtain the total consumed charge during the idleand stimulation periods. Alternatively, separate registers may directlymaintain a running tally of the charge depleted during stimulationperiods and idle periods, respectively.

Turning now to FIG. 7, a more detailed flow chart depicting thecalculation of the time to the end of service (EOS) and/or electivereplacement indicator (ERI) signals, as indicated in step 430 of FIG. 4,is illustrated. The IMD 200 is programmed for delivering to the patientelectrical pulses having predetermined parameters (step 710).Programming the stimulation settings may be performed duringmanufacturing and/or by a healthcare provider when the external unit 270gains communication access to the IMD 200. Occasionally, medicalpersonnel may determine that an alteration of one or more of thestimulation parameters is desirable. Implementation of such changes mayeasily be accomplished to optimize the therapy delivered by the IMD.Alternatively, as part of a routine diagnostic process, a predeterminedchange to the stimulation settings may be performed. Additionally, theIMD 200 may have multiple sets of stimulation parameters stored inmemory and may switch between the different stimulation modesrepresented by those parameters at preset times or at the occurrence ofcertain physiological events. When a change in one or more stimulationparameter settings is implemented (whether by programming or accessingdata from memory), the IMD 200 and/or the external unit 270 maydetermine an updated stimulation period current usage rate r_(s)associated with the new parameter settings, and subsequent updates tothe total charge consumed will be based upon the new stimulation periodcurrent usage rate (step 720). The rates may either be stored in memoryor calculated from an equation by interpolation among known currentrates for known parameter settings, as previously described. It is alsopossible that changes to the software or firmware of the device couldchange the idle period depletion rate, in which event a new idle periodcurrent usage rate r_(i) may also be calculated and reflected insubsequent calculations of total charge depleted (step 720).

Because the duty cycle (on-time to off-time ratio) is also a programmedparameter, the present invention allows both the idle period currentusage rate (r_(i)) and the stimulation period current usage rate (r_(s))to be combined into a single rate for purposes of projecting futureenergy or charge depletion and calculating a time to EOS and/or ERI.This rate represents the total current usage rate (r_(t)) of the device(step 725). Following updates to the stimulation and/or idle periodcurrent usage rates r_(s) and r_(i), the updated rates are then used tocalculate a new total charge remaining Q_(r), by a method substantiallyas shown in FIG. 6 and previously described. Once the total chargeremaining is retrieved from memory, the remaining time to an activationof an EOS is calculated (step 730) by using the total depletion rater_(t) and the total charge remaining Q_(r) on the battery until EOS.More particularly, the time remaining is calculated by dividing theremaining charge by the total depletion rate as shown in Equation 6.t=Q _(r) /r _(t)  Equation 6

At a predetermined time period before the end of service of the batteryunit 210 is reached, an ERI signal, which may prompt the healthcareprovider and/or the patient to schedule elective replacement of anelectronic device, may be asserted to provide a warning. ERI istypically determined as simply a predetermined time, for example from 1week to 1 year, more typically 6 months, earlier than EOS. In analternative embodiment, the ERI signal may be defined as a particularcharge level remaining (Q_(ERI)) above the EOS charge, Q_(EOS). In thisembodiment, the time period remaining until the ERI signal could becalculated by dividing Q_(EOS) by the total depletion rate r_(t) andsubtracting the resulting time period from the time to EOS as calculatedin equation 6.

The time to EOS provides a warning to the healthcare provider and/orpatient that the energy or charge supply will be depleted very shortly.Therefore, the time to EOS is reported to the implantable medical device200 and/or to the external device 270 (block 740). The ERI is alsoreported to the implantable medical device 200 and/or to the externaldevice 270, which is then brought to the attention of the patient and/ora medical professional.

In addition to battery life, for diagnostic purposes the impedance ofthe various leads that deliver stimulation provided by the IMD 200 isalso of interest. Lead impedance measurements and known output currentsignal characteristics may be used to calculate consumed stimulationcharge. Sudden changes in lead impedance may indicate any of a number ofchanges in the operation of the implantable medical device 200. Changesin impedance may indicate that the leads delivering the stimulation havemoved or have been damaged, or that the patient's body where thestimulation was delivered may have changed in some way.

Turning now to FIG. 8, a block diagram is provided depicting in furtherdetail an embodiment of the stimulation unit 250 of FIG. 2. Thestimulation unit 250 of the IMD 200 comprises an op amp unit 820, whichmay comprise one or more operational amplifiers that are capable ofdelivering a controlled current signal for stimulation. In oneembodiment, the controlled current is a constant current or asubstantially constant current. The stimulation unit 250 may alsocomprise an amplifier control circuitry unit 810 that may containcircuitry and/or programmable logic to control the operation of the opamps 820. Additionally, the stimulation unit 250 may be coupled to leads122, which may comprise a pair of signal wires capable of delivering anelectrical signal to an electrode pair 125-1 and 125-2 (FIG. 1D) eachcoupled to a distal end of one of the leads 122. The leads 122 (and theelectrodes 125-1 and 125-2) are capable of providing a complete circuitbetween the implantable medical device 200 and the region of thebody/tissue to which the electrodes are attached, which may beapproximated as an equivalent impedance. Each lead 122 may comprise asingle strand wire or, more preferably, a multi-strand wire braided orotherwise coupled together as a single functional wire. Each of the twolead wires 122 in this embodiment is provided with a separate socket andconnector 116, as shown in FIG. 1C. In another embodiment, two leads 122may be combined into a single coaxial cable (as shown in FIGS. 1A and1D), with a single socket providing both coaxial connectors 116.

Embodiments of the present invention provide for utilizing the deliveryof a constant current signal for delivery of stimulation, andmeasurement of the impedance experienced by the leads 122. In apreferred embodiment, the controlled or constant current signal providedby the stimulation unit 250 is independent of the impedance experiencedacross the leads 122. For example, even if the impedance experiencedacross the leads 122 changes, the op amp 820, in conjunction with theamplifier control circuitry 810, adjusts to deliver a controlled orconstant current despite the change in the impedance experienced acrossthe leads 122.

Since a controlled, constant current is delivered despite variations inthe impedance across the leads 122, the voltage across the leadterminals provide an indication of the lead impedance. For example, ifthe nerve tissue to which the leads 122 are connected has an impedanceof 1000 ohms, a particular stimulation may call for a one milliampconstant current signal. In this case, even if a 5000 ohms impedance isexperienced across the leads 122, the stimulation unit 250 will stillprovide a one milliamp current. Hence, the power may vary but thecurrent remains constant. In other words, the op amp 820 will stabilizeitself utilizing various circuitry, including the amplifier controlcircuitry 810, to provide a constant current signal even if theimpedance experienced by the leads 122 varies during the period thesignal is provided. Therefore, using Ohm's Law, V=IR, a measurement ofthe voltage across the leads 122 will provide an indication of theactual impedance experienced by the leads 122.

Turning now to FIG. 9, a block diagram depiction of one embodiment ofthe impedance measurement unit 265 from FIG. 2 is provided. In oneembodiment, the impedance measurement unit 265 comprises a voltagemeasurement unit 910, an A/D converter (analog to digital converter) 920and an impedance calculation unit 930. The voltage measurement unit 910is capable of measuring or determining the voltage differential betweenthe terminals of the leads 122. The signal from the voltage measurementunit 910 is generally an analog signal, which may be sent to the A/Dconverter 920. The A/D converter 920, which preferably has beencalibrated prior to the operation of the IMD 200, will convert theanalog voltage measurement signal to a digital signal. In alternativeembodiments of the present invention the impedance measurement unit 265may be implemented without the use of the A/D converter 920 and stillremain within the scope of the present invention.

Although certain embodiments may be implemented without it, the A/Dconverter 920 may be beneficial for enhancing the resolution of thevoltage signal, thereby providing for enhanced analysis of the voltageacross the leads 122. Based upon the voltage across the leads 122, andthe constant current signal provided by the stimulation unit 250, theimpedance calculation unit 930 calculates the impedance by dividing thevoltage across the lead terminals 122 by the current delivered by thestimulation unit 250. The impedance calculation unit 930 may be ahardware unit, a software unit, a firmware unit, or any combinationthereof, which may be located in various portions of the IMD 200,including in the impedance measurement unit 265, in the stimulationcontroller 230, in the power source controller 220, or in any otherportion of the IMD 200.

In an alternative embodiment, the calculation described as beingperformed by the impedance calculation unit 930 may alternatively beperformed by the external unit 270, which may receive the signalrelating to the constant current stimulation signal and the measuredvoltage signal. One of the advantages of utilizing the embodimentsprovided by the present invention is that substantially any size of aconstant or controlled current stimulus signal may be used to performthe impedance measurement, thereby conserving battery power of theimplantable medical device 200. Accordingly, the smallest stimulationsignal that may reliably be provided by the stimulation unit 250 may beused to perform the impedance measurement. Thus, the impedancemeasurement may be made without imposing a significant charge depletionburden on the battery. Additionally, the impedance of the leads 122themselves is also accounted for when analyzing the impedance.Furthermore, the A/D converter 920 may be calibrated prior to theoperation of the implantable medical device 200, for example, during themanufacturing process.

Turning again to FIGS. 1A-1D, the leads 122 are shown connected totissue (e.g., nerve tissue 127) in a patient's body and to the IMD 200.The implantable medical device 200 may comprise a main body 112 (FIG.1A) in which the electronics described in FIG. 2 are enclosed. Coupledto the main body 112 is a header 114 (FIG. 1A) designed with terminalconnectors 116 (FIG. 1C) for connecting to leads 122. The main body 112may comprise a titanium case 121 and the header 114 may comprise abiocompatible polymer such as polyurethane or acrylic. The leads 122projecting from the header 114 may be attached to the tissue utilizing avariety of methods for attaching the leads 122 to tissue. A first end ofthe leads 122 is coupled to connector(s) 116 on the header 114, and adistal end is coupled to the tissue by electrodes 125-1 and 125-2, whichtogether provide a cathode and an anode (FIG. 1D). Therefore, thecurrent flow may take place from one electrode 125-1 to a secondelectrode 125-2 via the tissue, thereby delivering the stimulation.

The system illustrated in FIGS. 1A-1D may be viewed as an electricalcircuit that includes a current or voltage source (i.e., the battery 210of the IMD 200) being connected to an impedance (i.e., the equivalentimpedance of the tissue) via a pair of wires (i.e., the leads 122). Thetotal impedance connected to the IMD 200 includes the impedance of thelead wires 122 as well as the impedance across the terminals 116 of theleads 122 to the tissue. One of the biggest components of the impedanceexperienced by terminals 116 on the header 114, to which the leads 122are connected, is the impedance of the tissue. Therefore, if a break inany one portion of the lead wires 122 occurs (such as a break in one ormore strands of a multistrand wire), the impedance may risesignificantly, which may provide an indication that a break in the leadwire 122 has occurred.

Turning now to FIG. 10, a flowchart depicting steps for determining theimpedance experienced by the leads 122 of the IMD 200 is provided. Asshown in step 1010, stimulation is delivered by the IMD 200 to thetissue of the patient by one of a number of available stimulationdelivery modes, such as a constant current signal pulse (step 1010). Toconserve battery power, impedance may be determined using a smallmagnitude and/or short duration pulse. The resultant voltage inducedacross the leads 122 is measured (block 1020) upon delivery of thestimulation signal. Voltage measurement may be performed by a voltagemeasurement unit 910 (FIG. 9) during delivery of the stimulation currentsignal. The IMD 200 adjusts the time at which the voltage is measuredsuch that it occurs while the stimulation current signal is beingdelivered.

An analog-to-digital (A/D) conversion is preferably performed on thevoltage signal (block 1030). Although, embodiments of the presentinvention may be performed without utilizing an A/D converter 920, in apreferred embodiment an A/D converter 920 (FIG. 9) is used to provideprecise resolution of the voltage signal. The A/D converter 920 ispreferably calibrated prior to the conversion of the voltage signal fromanalog to digital. Finally, the impedance is calculated utilizing theamplitude of the current delivered for stimulation and the correspondingvoltage measurement, as shown in step 1040. The voltage resulting fromthe current signal delivered as stimulation is divided by the value ofthe current to arrive at the total impedance across the terminals 116 ofthe header 114 (FIGS. 1A-1D). In one embodiment, the predeterminedimpedance of the lead 122 itself may be subtracted to arrive at theimpedance across the lead terminals 116, which corresponds to theimpedance of the tissue 1030. Various operational adjustments to theoperation of the IMD 200 may be made based upon the determination of theimpedance across the terminals 116.

FIG. 11 provides a flowchart depicting the steps for performing thecalibration of the A/D converter 920 (FIG. 9). In a preferredembodiment, the calibration of the A/D converter 920 is performed priorto implanting the IMD 200 in the body of the patient, more preferablyduring the manufacturing process of the IMD 200. Referring to FIG. 11, apredetermined, known impedance is provided for the calibration of theA/D converter 920, as depicted in step 1110. The known impedance iselectrically connected across the two distal ends of leads 122 (whichmay or may not include electrode assembly 125), and the other ends ofthe lead wires 122 are connected to the terminals 116 of header 114(step 1220). With the leads 122 connected between the IMD 200 and theknown impedance, a constant current test signal is driven through thelead 122, through the known impedance, and back to the IMD 200 (step1130).

The constant current test signal may comprise a series of individualconstant current signals that may vary in duration of current amplitudefrom one signal to another in the series of test signals, provided thateach individual test pulse comprises a constant current. During thedelivery of each constant current test pulse to the known impedance, acorresponding voltage resulting from the driving of the constant currentis measured across the terminals 116 of the IMD 200 (step 1240). Thismeasurement of voltage at the terminals 116 allows a comparison to atheoretical indication of what the measurement should be by calculationfrom the known current being driven, and the known impedance across theleads 122. This theoretical voltage calculation value is then used withthe actual voltage measured across the terminals 116 to calibrate theA/D converter 920 (block 1150). Calibration of the A/D converter 920should provide improved accuracy for measurements subsequently processedby the A/D converter 920. In another embodiment, the calibration processmay be performed using multiple known impedances and correspondingresulting multiple measured voltages. Such a calibration over a range ofimpedances may provide further improved accuracy.

Utilizing embodiments of the present invention, a more accurateassessment of the status of the battery and the impedance experienced bythe leads 122 may be assessed, thereby providing better warnings to theuser and/or to a healthcare provider assessing the operations of the IMD200. Various end of service signals (EOS) and/or elective replacementindication (ERI) signals may be provided to indicate the status of theoperation of the IMD 200. Additionally, the impedance experienced by theleads 122 of the IMD 200 may be analyzed to assess the integrity of theleads 122 or any drastic changes in the tissue to which the stimulationsignal is provided.

The particular embodiments disclosed above are illustrative only, as theinvention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended to thedetails of construction or design herein shown, other than as describedin the claims below. The particular embodiments disclosed above may bealtered or modified and all such variations are considered within thescope and spirit of the invention. Accordingly, the protection soughtherein is as set forth in the claims below.

1. A method for determining a time period until an end of service of anenergy storage device in an implantable medical device, said energystorage device comprising a total available electrical charge that maybe obtained from said energy storage device, said method comprising:determining a first current usage rate for a first future current usageperiod of an implantable medical device; determining a second currentusage rate for a second future current usage period of an implantablemedical device; determining a future combined current usage rate basedupon said first future current usage rate and said second current usagerate; and determining a total charge depleted by said implantablemedical device by determining a charge depleted during at least a firstprior current usage period, and determining a charge depleted during atleast a second prior current usage period; and determining a time perioduntil an end of service of the energy storage device based upon saidtotal available electrical charge, said total charge depleted, and saidfuture combined current usage rate.
 2. The method of claim 1, whereinsaid energy storage devices comprises a battery.
 3. The method of claim1 wherein said first prior current usage period comprises a stimulationperiod.
 4. The method of claim 1 wherein said second prior current usageperiod comprises an inactive period.
 5. The method of claim 1, furthercomprising the step of determining a third current usage rate for athird future current usage period of an implantable medical device, andwherein said step of determining a future combined current usage ratecomprises determining a future combined current usage rate based uponsaid first future current usage rate, said second current usage rate,and said third current usage rate.
 6. The method of claim 5 wherein saidfirst future current usage period comprises an inactive period, saidsecond future current usage period comprises a stimulation period, andsaid third future current usage period comprises a stimulation period,and wherein said second future current usage rate is different from saidthird current usage rate.
 7. A method for determining the remaininguseful life of an energy storage device in an implantable medicaldevice, said energy storage device having a total available electricalcharge that may be obtained from said energy storage device, said methodcomprising: determining an active charge depletion of the implantablemedical device; determining an inactive charge depletion of saidimplantable medical device; and determining a time period until an endof service of the energy storage device based upon said active chargedepletion, said inactive charge depletion, and said total availablecharge.
 8. The method of claim 7, wherein determining an active chargedepletion of an implantable medical device further comprises determininga charge depleted during a previous stimulation performed by saidimplantable device.
 9. The method of claim 8, wherein determining anactive charge depletion of an implantable medical device furthercomprises determining an active current usage rate for a futurestimulation to be performed by said implantable device.
 10. The methodof claim 9, wherein determining an active current usage rate for afuture stimulation to be performed by said implantable medical devicefurther comprises correlating a first predetermined current usage ratewith a first stimulation performed by said implantable device.
 11. Themethod of claim 10, wherein correlating said first predetermined currentusage rate with said first stimulation performed by said implantabledevice further comprises calibrating said first predetermined currentusage rate based upon at least one stimulation parameter.
 12. The methodof claim 10, wherein correlating said first predetermined current usagerate with said first stimulation performed by said implantable devicefurther comprises calibrating said first predetermined current usagerate based upon a lead impedance.
 13. The method of claim 10, whereindetermining an active current usage rate for a future stimulation to beperformed by said implantable medical device further comprisescorrelating a second predetermined current usage rate with a secondstimulation performed by said implantable device.
 14. The method ofclaim 13, wherein correlating said second predetermined current usagerate with said second stimulation performed by said implantable devicefurther comprises calibrating said second predetermined current usagerate based upon at least one stimulation parameter.
 15. The method ofclaim 7, wherein said energy storage device comprises a battery.
 16. Themethod of claim 7, wherein determining an inactive charge depletion ofan implantable medical device further comprises determining a chargedepleted during a previous idle period of said implantable medicaldevice.
 17. The method of claim 7, wherein determining an inactivecharge depletion of said implantable medical device further comprisesdetermining an idle current usage rate for a future idle period of saidimplantable medical device.
 18. The method of claim 17, furthercomprising calibrating said idle current usage rate for a previous idletime period.
 19. The method of claim 7, further comprising generating anend of service signal based upon a determination that said time perioduntil said end of service equals zero.
 20. The method of claim 7,further comprising generating an end of service signal based upon adetermination that said end of service has already occurred.
 21. Themethod of claim 20, further comprising generating an electivereplacement indicator signal based upon a determination that said timeperiod until said end of service is less than or equal to apredetermined period.
 22. The method of claim 21, wherein saidpredetermined period is six months.
 23. A method for determining a timeperiod until an end of service of an energy storage device in animplantable medical device, said energy storage device comprising aninitial charge corresponding to a substantially full charge and a finalcharge corresponding to an end of useful life, comprising: determiningan idle current usage rate for at least one future idle period of animplantable medical device; determining charge depleted during at leastone previous idle period; determining a stimulation current usage ratefor at least one future stimulation to be performed by said implantablemedical device; determining charge depleted during a previousstimulation performed by said implantable medical device; determining atotal charge depleted by said implantable medical device based upon saidcharge depleted during said previous idle period and said chargedepleted during said previous stimulation by said implantable medicaldevice; determining a future combined current usage based upon said idlecurrent usage rate and said stimulation current usage rate; anddetermining a time period until an end of service of the energy storagedevice based upon said total charge depleted, said future combinedcurrent usage rate, said initial battery charge and said final batterycharge.
 24. The method of claim 23, wherein said method furthercomprises determining a previous idle current usage rate and a previousstimulation current usage rate, and wherein said step of determiningcharge depleted during a previous idle period comprises multiplying saidprevious idle current usage rate by the duration of said previous idleperiod, and said step of determining charge depleted during a previousstimulation comprises multiplying said previous stimulation currentusage rate by the duration of said stimulation.
 25. The method of claim23, further comprising generating an end of service signal based upon adetermination that said time period until said end of service equalszero.
 26. The method of claim 25, wherein further comprising generatingan elective replacement indicator signal based upon a determination thatsaid time period until said end of service is less than or equal to apredetermined period.
 27. The method of claim 26 wherein saidpredetermined period is six months.
 28. A method for determining theremaining useful life of a battery in an implantable medical device,said battery having a first charge and a second charge less than saidfirst charge comprising: determining a total available charge for saidbattery comprising the difference between said first charge and saidsecond charge; determining a charge depletion of the implantable medicaldevice; and determining a time period until an end of service of a powersupply associated with said implantable medical device based upon saidcharge depletion and said total available charge.
 29. A method fordetermining a remaining time until the end of service of a battery of animplantable medical device, said battery comprising a total availableelectrical charge that may be obtained from said battery, said methodcomprising: determining a previous active charge depletion of animplantable device; determining a future active current usage rate ofsaid implantable device; determining a previous inactive chargedepletion of said implantable device; determining a future inactivecurrent usage rate of said implantable device; determining a remainingtime period until an end of service of said battery based upon saidtotal available electrical charge, said previous active chargedepletion, said future active current usage rate, said previous inactivecharge depletion, and said future inactive current usage rate.
 30. Themethod of claim 29, further comprising the step of generating an end ofservice signal based upon a determination that said time period untilsaid end of service equals zero.
 31. The method of claim 29, furthercomprising the step of generating an end of service signal based upon adetermination that said end of service has already occurred.
 32. Themethod of claim 29, further comprising the step of generating anelective replacement indicator signal based upon a determination thatsaid time period until said end of service is less than or equal to apredetermined period.
 33. The method of claim 30, wherein saidpredetermined time period is six months.
 34. An implantable medicaldevice, comprising: an energy storage device to provide power for anoperation performed by said implantable medical device, characterized bya total available electrical charge defined by the difference between aninitial electrical charge and a final electrical charge; a stimulationunit, operatively coupled to said energy storage device for providing astimulation signal; and a controller operatively coupled to saidstimulation unit and said energy storage device, said controllercomprising: an active charge depletion determination unit adapted todetermine an electrical charge depleted by said energy storage deviceduring stimulation operations of the implantable medical device; aninactive charge depletion determination unit adapted to determine anelectrical charge depleted by said energy storage device during inactiveperiods of the implantable medical device; and an end of servicedetermination unit adapted to determine a time period until an end ofservice of said energy storage device based upon said total availableelectrical charge, said active charge depletion, and said inactivecharge depletion.
 35. The implantable device of claim 34, wherein saidimplantable device is a vagus nerve stimulator device.
 36. Theimplantable device of claim 34, wherein said active charge depletiondetermination unit is adapted to determine an active charge depletionrelating to at least one previous stimulation period performed by saidimplantable device.
 37. The implantable device of claim 34, wherein saidactive charge depletion determination unit is adapted to determine anactive current usage rate for a future stimulation period to beperformed by said implantable medical device
 38. The implantable medicaldevice of claim 34, wherein said inactive charge depletion determinationunit is adapted to determine an idle charge depletion relating to atleast one previous idle period of said implantable medical device. 39.The implantable medical device of claim 34, wherein said inactive chargedepletion determination unit is adapted to determine an idle currentusage rate for a future idle period of said implantable medical device.40. The implantable medical device of claim 34, wherein said controlleris adapted to generate an end of service signal based upon adetermination that said time period until said end of service equalszero.
 41. The implantable medical device of claim 34, wherein saidcontroller is adapted to generate an elective replacement indicatorsignal based upon a determination that said time period until said endof service is less than or equal to a predetermined period.
 42. Theimplantable medical device of claim 41, wherein said predeterminedperiod is six months.
 43. A system for determining remaining useful lifeof a battery in an implantable medical device, comprising: an externalmonitoring device for performing remote communications with animplantable medical device; an implantable medical device adapted tocommunicate with said external device and to deliver a stimulationsignal to a patient, said implantable device comprising: a battery toprovide power for an operation performed by said implantable medicaldevice; a communications unit to provide for communications between saidexternal monitoring device and said implantable medical device; astimulation unit, operatively coupled to said battery, to provide astimulation signal; and a controller operatively coupled to saidstimulation unit and said battery, said controller being adapted todetermine: an active charge depletion of said implantable medicaldevice; an inactive charge depletion of said implantable medical device;and a time period until an end of service of said battery based uponsaid active charge depletion and said inactive charge depletion.
 44. Thesystem of claim 43, wherein said controller further comprises an activecharge depletion determination unit to determine an active chargedepletion relating to a previous stimulation period performed by saidimplantable medical device.
 45. The system of claim 44, wherein saidcontroller further comprises an inactive charge depletion determinationunit to determine an inactive charge depletion relating to a previousinactive period of said implantable medical device.
 46. The system ofclaim 45, wherein said controller further comprises an EOS/ERIdetermination unit to determine said time period until an end of serviceof said battery based upon said active charge depletion and saidinactive charge depletion.
 47. The system of claim 46, wherein at leastone of said active charge depletion determination unit, said inactivecharge depletion determination unit, and said EOS/ERI determination unitcomprises of at least one of hardware, software, firmware, and acombination of said hardware, software and firmware.
 48. The system ofclaim 43, wherein said controller is adapted to determine an activecurrent usage rate relating to a potential stimulation to be performedby said implantable device
 49. The system of claim 43, wherein saidcontroller is adapted to determine an idle current usage rate relatingto a potential idle period of said implantable medical device.
 50. Thesystem of claim 43, wherein said controller is adapted to generate anend of service signal based upon a determination that said time perioduntil said end of service equals zero.
 51. The system of claim 43,wherein said controller is adapted to generate an end of service signalbased upon a determination that said end of service time has alreadyoccurred.
 52. The system of claim 43, wherein said controller is adaptedto generate an elective replacement indicator signal based upon adetermination that said time period until said end of service is lessthan or equal to a predetermined period.
 53. The system of claim 52,wherein said predetermined period is 6 months.
 54. A computer readableprogram storage device encoded with instructions that, when executed bya computer, performs a method, comprising: determining an active chargedepletion of an implantable medical device; determining an inactivecharge depletion of said implantable medical device; and determining atime period until an end of service of an energy storage deviceassociated with said implantable medical device based upon said activecharge depletion and said inactive charge depletion.
 55. The computerreadable program storage device of claim 54, wherein determining anactive charge depletion of an implantable medical device comprisesdetermining the charge depleted during a previous stimulation periodperformed by said implantable medical device.
 56. The computer readableprogram storage device of claim 54, wherein determining an active chargedepletion of an implantable medical device comprises determining anactive current usage rate for a future stimulation period to beperformed by said implantable medical device.
 57. The computer readableprogram storage device of claim 56, wherein determining an activecurrent usage rate for a future stimulation period to be performed bysaid implantable medical device further comprises correlating a firstpredetermined current usage rate with a first stimulation performed bysaid implantable device.
 58. The computer readable program storagedevice of claim 57, wherein correlating said first predetermined currentusage rate with said first stimulation performed by said implantabledevice further comprises calibrating said first predetermined currentusage rate based upon at least one stimulation parameter.
 59. Thecomputer readable program storage device of claim 57, whereindetermining an active current usage rate for a future stimulation periodfurther comprises correlating a second predetermined current usage ratewith a second stimulation performed by said implantable device.
 60. Thecomputer readable program storage device of claim 59, whereincorrelating said second predetermined current usage rate with saidsecond stimulation performed by said implantable device furthercomprises calibrating said second predetermined current usage rate basedupon at least one stimulation parameter.
 61. The computer readableprogram storage device of claim 54, wherein determining an inactivecharge depletion of an implantable medical device comprises determiningthe charge depleted during a previous inactive period of the implantablemedical device.
 62. The computer readable program storage device ofclaim 61, wherein determining an inactive charge depletion of animplantable medical device comprises determining an inactive currentusage rate for a future inactive period of the implantable medicaldevice.
 61. The computer readable program storage device of claim 62,further comprising calibrating said inactive current usage rate for aprevious inactive period.
 62. The computer readable program storagedevice of claim 54, further comprising generating an end of servicesignal based upon a determination that said time period until said endof service equals zero.
 63. The computer readable program storage deviceof claim 54, further comprising generating an elective replacementindicator signal based upon a determination that said time period untilsaid end of service is less than or equal to a predetermined period. 64.The computer readable program storage device of claim 65, wherein saidpredetermined period is six months.